Non-destructive microwave radar finds moisture hiding in the walls of a building

For homeowners, moisture buildup can be the biggest headache. Mold grows on drywall and wood materials and crawls along walls, floors and ceilings. Building materials begin to erode and rot. When the insulation becomes damaged, the energy efficiency of the home decreases. Even human health suffers as humidity also causes air quality problems.

The key to preventing extensive moisture damage is to detect it early, when it can be easily remedied.

Researchers at the Department of Energy's Oak Ridge National Laboratory use microwave radar reflection to nondestructively detect and measure the moisture content of materials in walls without removing drywall or paneling. This also speeds up moisture detection and allows early stage mold growth to be controlled.

The research team's study results were published in IEEE Xplore and presented during the 2024 IEEE Radar Conference in Denver.

“We know that microwave radar is promising for this because it is known to be able to measure moisture in wood samples,” said ORNL's Philip Boudreaux. “But can it measure the moisture in the wood within a wall to detect high moisture problems before they become a major problem? That’s the challenge.”

A leaky seal

The building envelope consists of external walls, the roof and the foundation, which are connected together to prevent the transfer of moisture. But the casing itself is susceptible to moisture problems caused by many factors: too much rain, soil moisture, air escaping through holes, and vapor diffusion as moisture moves through the casing from higher to lower concentrations.

Most homes are made of wood frame structures, and when wood is damp, it is the perfect environment for mold to develop. If a wall is damaged or improperly designed, the water vapor penetrating through the wood can cause it to become damp. For this reason, says Boudreaux, wood was chosen as the starting material for studying the performance of microwave radar.

“You can detect water in wood using microwave energy reflected from the material using radar,” Boudreaux said. “You can also measure moisture in more than one type of material within the wall.”

As part of the electromagnetic spectrum, microwaves interact with materials in a similar way to visible light, but penetrate further and create reflections. Radar systems emit signals such as microwaves and then detect the reflections of those microwaves. When used on walls, the properties of the microwave reflection pulse are based on the moisture in the material.

Walls are made up of layers of material, and each layer can contain different amounts of moisture. However, by measuring the time it takes for the microwaves to return to the sensor, the distance to each material in the wall can be calculated, and this can be used to image and measure the moisture within the layers.

Re-measure

ORNL's study focused on detecting moisture in the wall's structural sheathing, which in homes is commonly OSB board. This board sits directly behind the cladding or exterior layer of wood frame construction.

Boudreaux and his team conducted experimental tests on 305-millimeter wood sheathing samples that were 10 millimeters thick. Each piece was dried in an oven and then conditioned to achieve a specific moisture content. The team used microwave radar to measure the moisture content of the cladding and then compared the results with measurements taken with a traditional hand-held moisture meter.

“The results proved that microwave radar technology can detect and measure moisture within the jacket with an accuracy of 3% compared to traditional hand-held meters,” he said. In order for the technology to work in buildings, Boudreaux's team first had to figure out whether the microwave radar could detect the sheathing behind the drywall.

“So we placed a piece of drywall 8.5 centimeters in front of the sheathing and immediately saw that the radar picked up the sheathing,” he said.

The proof-of-concept test then moved on to the second phase: determining whether the radar could detect the moisture content of the casing. By applying mathematical algorithms developed by the ORNL research team, the results were once again positive. The radar reflection signals could be correlated with the moisture content.

“We are able to predict the shape of the reflected microwave pulse from wet chipboard,” he said. “The pulse can also be analyzed empirically by relating the pulse characteristics to the moisture content.”

Next, the researchers converted the experimental setup into a portable, custom-made miniature electronic system to enable field measurements. The experimental setup included measurements based on the reflection of microwave signals transmitted over a frequency of 10-15 gigahertz.

Beyond walls

Based on the research team's promising results, the plan is to license the ORNL technology to a manufacturer so that the portable microwave radar system can one day be purchased off the shelf and allow any inspector or homeowner to purchase and use the device.

The tool could also be used to evaluate roofs and foundations, helping potential home buyers avoid surprises by identifying problems that might go unnoticed during an inspection.

“In developing the detector, we designed a system specifically designed for residential walls and general public access,” said Boudreaux. “It is small, portable, lightweight, easy to set up and can be adapted to transmit within frequency regulations.”

The team's next phase of research involves measuring microwave reflectance from complete wall assemblies with different claddings such as vinyl siding and brick.

“With early detection, a small problem can be addressed before major damage occurs,” he said. “We've figured out what can help locate this moisture early, but we still have a lot of work to do and need to test more materials and explore further frontiers with microwave radar reflection.”